The invention relates to a disc brake having two brake shoes, which for generating a clamping force are pressable against both sides of a brake disc, and having an actuator device for actuating at least one of the brake shoes. The invention further relates to a method of effecting open- or closed-loop control of a brake system comprising such a disc brake.
A disc brake of the described type is known from WO 99/05011, and corresponding U.S. Pat. No. 6,394,235 which is incorporated by reference herein. The forces arising in said disc brake during a braking operation may be subdivided into clamping force (also known as axial force, transverse force, brake application force or normal force) and peripheral force (also known as frictional force). The component of force introduced by a brake shoe into the brake disc at right angles to the plane of the brake disc is described as a clamping force. By peripheral force, on the other hand, is meant the component of force, which on account of the brake friction between a friction lining of the brake shoe and the brake disc acts in peripheral direction of the brake disc upon the brake shoe. By multiplying the peripheral force by the distance of the application point of the peripheral force from the axis of rotation of the wheels, the braking torque may be determined.
In the disc brake known from WO 99/05011, the clamping force is generated either hydraulically or by means of an electric motor. In the case of hydraulic clamping force generation, a pressurized hydraulic fluid is introduced into a hydraulic chamber formed in a housing of the disc brake. A hollow piston of an actuator device accommodated displaceably in the hydraulic chamber is moved by the hydraulic fluid in the direction of one of the two brake shoes and brings it into frictional engagement with the brake disc. Since the disc brake is designed as a floating-caliper disc brake, in a known manner the brake shoe not interacting directly with the piston is also pressed against the brake disc.
In the case of clamping force generation by means of a motor, the rotational movement of a motor shaft is first stepped down by means of a planetary gear and then converted into a translational movement by means of a nut/spindle arrangement of the actuator device that is disposed inside the hollow piston. The hollow piston is seized by this translational movement and transmits the translational movement to one of the two brake shoes, which is subsequently pressed against the brake disc.
Future brake systems, for open- and closed-loop control purposes, require an exact acquisition of the forces arising during a braking operation. It is therefore customary to equip disc brakes with one or more force sensors and to connect these force sensors to open- and closed-loop control circuits.
In WO 99/05011 it is proposed to improve control of the disc brake by providing a force sensor, which detects the clamping force and may be disposed e.g. in or at a brake shoe or alternatively in the hollow piston. By means of such a force sensor the clamping force may be detected over a wide clamping force range and a suitable control signal may be generated.
In DE 196 39 686 A1, and corresponding U.S. Pat. No. 6,059,379 which is incorporated by reference herein, a disc brake equipped with force sensors is likewise described. The disc brake possesses two force sensors, which are disposed in each case on a fastening screw, by means of which a caliper is connected to a vehicle-fixed holder. The force sensors are used to detect the peripheral force, which a control device of an electromechanical wheel brake actuator, which is not described in detail, takes into account during setting of the clamping force.